Apparatus for and method of emitting particles



y 1966 J. M. HAMMER 3,253,402

APPARATUS FOR AND METHOD OF EMITTING PARTICLES Filed Jan. 29, 1963 s Sheets-Sheet 1 F g I /o/v saver: 14 2! 10 I j 6 53- 3212 K M1zuFaqzaqqqzofiza g- U 22 WE I I I I I I I I I f |||I||||||]|||||L||||||||||I||Il IN VEN TOR.

J4me M HAM/145? BY Arron/[r May 31, 1966 J. M. HAMMER APPARATUS FOR AND METHOD OF EMITTING PARTICLES 5 Sheets-Sheet 2 Filed Jan. 29, 1963 f 6/15 M/ZET /a/v Sal/K65 6 45 m lil IN VENTOR. Jhc'oa M hQMMA-"K 4rm6vr y 1966 J. M. HAMMER 3,253,402

APPARATUS FOR AND METHOD OF EMITTING PARTICLES Filed Jan. 29, 1965 3 Sheets-Sheet 5 INVENTOR. [4605 M hQ/MA/i? United States Patent 3,253,402 APPARATUS FOR AND METHOD F EMITTTN G PARTICLES Jacob M. Hammer, Lawrence Township, Mercer County,

NJ, assignor to Radio Corporation of America, a corporation of Delaware Filed Jan. 29, 1963, Ser. No. 254,813 Claims. (Cl. 60-355) The present invention relates to apparatus for and methods of emitting particles, and particularly to apparatus for and methods of providing a beam of high speed neutral atomic particles.

The invention is especially suitable for use in a propulsion device, such as an electric rocket engine for space vehicles. The invention may also be used to provide beams of particles for aerodynamic, atomic and nuclear research.

Ion rocket engines for vehicles have been proposed which produce ions, accelerate these ions, and emit them at high speed to provide thrust. The problem remains of neutralizing the ions to prevent the vehicle from acquiring a charge which might prevent the emission of further ions. Also, the energy required to produce each ion is lost so far as the propulsive efliciency of the ion engine is concerned. The ion engine therefore has low efficiency except where the energy expended to produce the ions is negligible as compared to; the energy expended to accelerate the ions to high velocity. High efficiency dictates that a relatively low density beam of ions be emitted at high velocity. For many purposes, a lower velocity, denser particle beam is desirable.

The time required for a vehicle to reach a specified velocity from rest, which also is a measure of the effectiveness of an engine to convert propulsive energy into jet kinetic energy, is known as specific impulse, 1 The specific impulse is also defined as the ratio of the velocity of the exhaust jet to the acceleration of gravity. The longer the specific impulse, the less effective is the engine in accelerating the vehicle. The efiiciency of an ion engine decreases with decreasing specific impulse. Another limitation of proposed ion engines is relatively low specific thrust (i.e., thrust in the direction of the ion beam per unit area in a plane perpendicular to the direction of the ion beam). Space charge effects prevent large ion density in spite of large accelerating voltages.

Another proposed electric rocket engine is the plasma engine. These plasma engines mainly use electrical pulses to accelerate the plasma discontinuously and have undesirably low duty cycle and undesirably low average specific thrust.

Accordingly, it is an object of the present invention to provide improved methods of and apparatus for particle acceleration which reduce or eliminate the foregoing difiiculties and disadvantages.

It is a further object of the present invention to provide improved methods of and apparatus for electric rocket propulsion especially suitable for space vehicles.

It is a still further object of the present invention to provide improved methods of and apparatus for ion acceleration which methods and apparatus produce an efilux of high speed neutral particles, thereby eliminating the need for ion neutralization.

It is a still further object of the present invention to provide an improved electric rocket engine which can operate more efficiently than do electric rocket engines proposed heretofore, particularly at specific impulses below 5,000 seconds.

It is a still further object of the present invention to provide improved methods of and apparatus for providing a denser beam of high speed particles than can be 7 3,253,402 Patented May 31, 1966 achieved with known particle acceleration techniques, particularly with ion acceleration techniques.

It is a still further object of the present invention to provide improved methods of and apparatus for electric propulsion with larger effective specific thrust than with known electric propulsion techniques.

It is a still further object of the invention to provide improved methods of and apparatus for producing beams of high speed atomic particles suitable for aerodynamic, atomic or nuclear research.

It is a still further object of the present invention to provide methods of and apparatus for emitting a beam of atomic particles which does not contain significant quantities of unbound charged particles, which beam is not electrically unstable and is not of such a nature as may cause interference with communication.

The foregoing objects and advantages may be obtained, according to the invention, by repetitively causing charge exchange interactions between charged particles (such as an ionized particle) and neutrals (that is, uncharged particles, such as atoms or molecules), in the presence of a field which tends to accelerate the charged particles. The charged particle is preferably of the same species as the neutral, that is, the charged particle is that particle obtained by removing from or adding to the neutral, one o r more electrons. For example, an ion and atom of the same element are of the same species, or an ionized and neutral molecule of the same compound are of the same species.

Each charge exchange between a fast ionized particle and a slow neutral converts (1) the ionized particle into a fast neutral, moving in the same direction as the fast ionized particle, and (2) the neutral into a slow ionized particle which is, in turn, accelerated by the field. A cascade of such charge exchange interactions may occur for each ionized particle thereby providing many fast neutrals per initial ionized particle. These neutrals may be ejected, for example, in a beam for propulsion or for other purposes, as in research.

The invention itself, both as to its organization and method of operation, as well as additional objects and advantages thereof, will become more readily apparent in connection with the accompanying drawings, in which:

FIG. 1 is a schematic view of particle acceleration apparatus embodying the invention,

FIG. 2 is a diagram illustrating the operation of the apparatus shown in FIG. 1,

FIG. 3 is a sectional view of an electric rocket engine embodying the invention, the view being taken along the line 3-3 in FIG. 4, viewed in the direction of the appended arrows,

FIG. 4 is a sectional view of the engine shown in FIG. 3, taken along the line 4-4 in FIG. 3, viewed in the direction of the appended arrows,

FIG. 5 is a sectional view of a multiple beam, electric rocket engine embodying the invention, the view being taken along the line 5-5 in FIG. 6, viewed in the direction of the appended arrows, and

FIG. 6 is a sectional view of the engine shown in FIG. 5, taken along the line 6-6, viewed in the direction of the appended arrows.

The following theoretical discussion is presented by way of introduction to the more detailed aspects of the invention. In theory, there is a finite probability that an ionized particle, such as an ion, which passes through the vicinity of a neutral, such as an atom (sometimes called making a distant collision), captures an electron from that atom and leaves the atom ionized. This effect is called ionization by charge exchange, or simply charge exchange, and was referred to above. When the atom and ion involved in the collision are of the same species,

the effect is called resonance charge exchange. In resonance charge exchange, both energy and momentum are conserved. The emerging atom has almost the same energy and directed momentum as the incident ion, and the remaining ion has the same energy and momentum as the original atom. Since resonance charge exchange is the more efficient process, it preferably is used in practicing the invention. However, charge exchange of a non-resonant character between atoms and ions, respectively, of suitable, different species, may be used for the initial charge exchange event. Subsequent exchanges will be resonant.

The probability of occurrence of a charge exchange is directly related to the so-called charge exchange crosssection Q The cross-section for ion-atom charge exchange is, by far, the most dominant collision process among ions, atoms and electrons at ion speeds in the range of interest for specific impulses below 5,000 seconds, and especially for certain elements, such as the noble gases. The probability that an ion may collide (make a close encounter) with a slow neutral atom and be scattered without capturing an electron is related to the ion scattering cross-section, Q

The probability that a fast moving neutral, such as results from a charge exchange interaction, may collide (make a close encounter) with a slow neutral and be scattered so that the fast neutral does not effectively contribute to the thrust is related to the fast neutral scattering cross-section, Q 'Q is larger than either Q or Q particularly for particle velocities corresponding to specific impulses below 5,000 seconds, as indicated by Table I. This table presents values of the aforementioned cross-sections for xenon and helium at certain specific impulses, ion velocities and ion energies, by way of example. is related to the cross-section for charge exchange in accordance with the relation,

where N is the number of the neutrals per cubic centimeter.

The mean free path for ion-slow neutral scattering, h and for fast-neutral, slow-neutral scattering, M may be found from similar relationships. The mean free paths for charge exchange, as may be observed by applying the above formula using the data in Table I, is much smaller than either scattering mean free paths M or h Thus an ion charge exchanges with a slow neutral long before it has an appreciable probability of being scattered. Also, a fast neutral may leave a region containing slow neutrals long before that fast neutral has an appreciable probability of being scattered.

The mean free path for charge exchange the wall of the aperture between the forwardmost electrode and the ion source 12 for admitting a neutral gas into the envelope 10. Fields may be established by means of electrodes and/or electromagnets in the ion source 12 to ionize the gas flowing into the source 12 through the inlet 16. Voltages may be applied to an apertured electrode or grid 18 at the end of the source 12 which faces the electrode structure 14 so that the grid 18 attracts positive ions. 1 The positive ions pass through the grid 18 into the envelope 10.

Another suitable type of ion source 12 may include an electron gun which shoots electrons through a gas, thereby ionizing the gas. Ion sources of this type are described in the following publications: Journal of Applied Physics, vol. 29, No. 3, pp. 549555, March 1958; and Review of Scientific Instruments, vol. 11 (1940), p. 94, et seq. Still other ion sources of the type which utilize an electric are or glow discharge to produce a plasma of ionized gas may also be suitable. Alternatively, ion sources which emit alkali (e.g., cesium) ions by ionization of alkali atoms on hot surfaces may be employed. In that case, an alkali gas, such as cesium vapor, may be admitted through the inlet 24, and the inlet 16 omitted, if desired.

The electrode structure 14 includes a plurality of centrally apertured disc electrodes 20. Eleven such disc electrodes 20a to 20k are shown by way of illustration, these being spaced successively and at equal distances from each other along the envelope 10. All of the electrodes 20 are coaxial with each other and may be obaxial with the ion source 12 and the envelope 10, so that the ion beam emitted by the source 12 travels axially in the envelope 10 along a channel defined by the apertures in the electrodes 20. The electrode 20k nearest the rear end 13 of the envelope 10 is connected to, or may be integral with, a cylindrical tube 22 which extends to the rear end 13 of the envelope 10. The tube may have an inside diameter equal to the diameter of the aperture in the rear end 13 of the envelope 10 and to the apertures in the electrodes 20. The axial length of the tube 22 is preferably greater than the axial distance occupied by the discs 20a-20k of the electrode structure 14 so as to retard the escape of the neutral gas from the envelope 10 through the rear end aperture 13, as by diffusion.

The electrodes 20 are connected to successive taps of a source of direct current voltage illustrated as a battery 28. The positive terminal of the battery 28 is connected to the forwardrnost electrode 20a. The negative terminal of the battery is connected to the rearwardmost electrodes 20k. The negative terminal of the battery may also be connected to a source of reference potential, such as ground, or the body of the vehicle.

Operation of the devices described herein' does not depend, however, on the accuracy or acceptability of any theory, and the foregoing discussion is intended only to engender a better understanding of the invention.

Referring to FIG. 1, there is shown an envelope 10 containing an ion source 12 and an electrode structure 14. The envelope may be a cylindrical tube having a forward end 11 and a rear end 13. The forward end 11 includes a gas inlet 16. The rear end 13 has a central aperture 15. The ion source 12 may be an ion gun of the type known in the art which emits positive ions. The ion source 12 may be supplied with gas to be ionized Progressively increasing negative voltages are applied to successive ones of the electrodes 20a-20k so as to establish an electric field for accelerating the positive ions emitted by the ion source 12 toward the rear end 13 of the envelope 10. The voltage difference between succeeding ones of the electrodes 20 is equal, and is of a magnitude which depends upon the type of gas which enters the envelope 10 through the inlet 24. The spacing between successive electrodes is also desirably equal. Thus, the electric field gradient between adjacent electrodes is the same. The field between the disc electrodes 20 also tends to focus the positive ions into a beam which travels through the inlet 16. Another inlet 24 is provided in along the channel or path defined by the apertures in the electrodes 20. Instead of an electric accelerating field, as provided by the electrode structure 14, an electromagnetic accelerating field (for example, a traveling wave field) may be established, which field accelerates the ions. The accelerating electromagnetic field may be established by a coil system of the type which is used in linear accelerators to provide traveling electromagnetic waves.

When the apparatus shown in FIG. 1 is to be used in a space vehicle (for example, more than 200 miles from the surface of the earth), the aperture 15 will be in communication with the vacuum which exists in outer space. Accordingly, the envelope is evacuated. A vacuum pump system may be disposed in vacuum tight relationship with the forward end 13 of the envelope 10 for evacuating the envelope when the apparatus is used near ground level. For example, when the apparatus is used in a laboratory as a research tool to provide high speed atomic particles, the envelope is desirably evacuated by a suitable vacuum pumping system. The vacuum and the gas inlet pressure may be adjusted to provide a predetermined pressure of the neutral gas and ions which are admitted into the envelope 10. It may be desirable to provide a heater element around the envelope 10 for controlling the pressure of the gas in the envelope by varying the temperature of the gas.

The progress of an ion which is emitted by the source 12 is illustrated in FIG. 2. It should be understood that this ion is merely representative of a distribution of ions and interaction events. The vector E indicates the direction of the accelerating field. This is in the direction axially of the envelope 10 toward the rear end 13 thereof. An ion I is emitted by the source 12 and is accelerated by the field. After traveling a distance depending on the mean free path for charge exchange, A the ion I collides with a neutral N which may be a slow neutral atom of the gas which is admitted into the envelope 10 through the inlet 24. This charge exchange interaction may take place between the foremost electrode 20a and its succeeding, adjacent electrode 20b. By virtue of this charge exchange, the ion I captures an electronand is converted into a fast neutral atom which travels in the same direction as the ion I along the channel defined by the apertures in the disc electrodes and the opening in the tube 22 and out of the rear end 13 of the envelope 10. Because the ion I is focused by the electric field established in the electrode structure 14 to travel axially along the channel defined by the electrodes 20 and tube 22, the fast neutral resulting from the charge exchange also travels along this channel and is emitted from the envelope 10.

The charge exchange between the ion I and the neutral N also results in a slow ion I This ion I is accelerated by the field between the first and second electrodes 20a and 20b and travels another distance depending on the mean free path for charge exchange, A whereupon this ion I charge exchanges with another neutral N to provide another fast neutral and another slow ion I The latter fast neutral travels down the channel and out of the envelope 10. Each fast neutral contributes to the thrust produced by the apparatus. The second charge exchange between the ion I and the neutral N may take place between the second and third electrodes 20b and 200. It may be desirable to provide a separation of approximately one mean free path for charge exchange A between succeeding adjacent electrodes 20. The slow ion 1 is accelerated, as was the slow ion I and charge exchanges between the third and fourth electrodes 20c and 20d with another neutral N to provide still another fast neutral, which is emitted from the envelope, and another slow ion I Since the adjacent disc electrodes are separated by one mean free path and since there are eleven (11) such disc electrodes 20, on an average, ten charge exchange interactions may occur as shown in FIG. 2. Accordingly, ten fast neut-rals are provided on the average for each ion which is emitted by the ion source 12. The slow ions resulting from the last of the charge exchanges may be collected on the walls of the tube 22. However, the slow ions may be accelerated and emitted from the envelope 10, so as to contribute to the total thrust provided by the apparatus, as desired.

Let it be assumed, solely for purposes of illustration, that the gas which is admitted through the inlet 16 to be ionized in the source 12 and the gas which is admitted through the inlet 24 is xenon. The pressure of the xenon may be six microns of mercury, so that the mean free path for charge exchange is approximately one centimeter. The disc electrodes 20 (including 20k) may be separated from each other by a distance of one centimeter. The axial distance between the first and last discs is ten cms. The tube 22 may be less than 14 cms. in length. The electric field gradient may be 500 volt per cm. Since the mean free paths for either ion scattering A or fast neutral scattering h is about 25 cms., the dominant process in the envelope will be charge exchange bet-ween the xenon positive ions and the xenon neutral atoms. The envelope is too small for appreciable ion, neutral scattering and fast neutral, slow neutral scattering. The apparatus has a cascading or multiplying factor of ions entering to atoms leaving the apparatus of approximately tens. The foregoing parameters are provided solely for the purpose of facilitating the understanding of the invention.

The apparatus illustrated schematic-ally in FIG. 1 may be constructed as shown in FIG. 3. A plurality of conductive, metal disc electrodes 30, 32, 34, 36 and 38 are arranged in coaxial relationship. The last of these electrodes 38 is formed integrally with a tube 40. The beam of neutrals is emitted from the open end 42 of this tube 40. The disc electrodes have circular openings 44 through their centers. alignment with each other and with the opening through the tube 40, and they define a channel for the ion and fast neutral beams.

While only five electrodes 30, 32, 34, 36 and 38 are shown, more or less electrodes may be used depending upon the multiplication or cascading factor desired.

Rings 46 of insulating material, such as a ceramic, separate adjacent ones of the disc electrodes. Vacuum tight seals may be made between the electrodes 30, 32, 34, 36 and 38- and the rings 46 by brazing techniques known in the electron tube assembly art.

An envelope 48 containing an ion source of the type described above in connection with FIG. 1 may be attachcd to the first electrode 30 by brazing or other technique for providing a vacuum tight seal. An inlet nozzle 50 for gas to be ionized enters the envelope 48. An opening 52 in the first ring 46 between the electrodes 30 and 32 receives a nozzle 54 through which the neutral charge exchange gas may be admitted. The internal parts of the ion source 48 may be constructed separately prior to assembly.

Operating potentials may be applied to the electrodes 30, 32, 34, 36 and 38 from a suitable voltage source, such as shown in FIG. 1. Similarly, sources of operating potential may also be connected to the ion source. Terminals on the ion source envelope 48 for applying the operating potentials have been omitted to clarify the i1- lustration. When gas is admitted through the nozzles 50 and 54 and suitable operating voltages are applied to the electrodes 30, 32, 34, 36 and 38, the apparatus operates to provide a beam of neutrals, as was explained in connection with FIGS. 1 and 2.

Apparatus for providing multiple beams of neutrals, which apparatus is especially suitable for use as an electric rocket engine, is shown in FIGS. 5 and 6. This apparatus includes a plurality of disc electrodes 60, 62 and 64, only three of which are shown to simplify the illustration. The rearwa-rdmost electrode 64 is formed These openings 44 are in series 4 with five cylindrical tubes. Three of these tubes 66, 68 and 70 are illustrated in FIG. 5. The electrodes 60, 62 and 64 each have five openings 72, 74, 76, 78 and 80 therein. The opening 72 extends through the center of the disc, and the other openings 74, 76, 78 and 80 are symmetrically disposed about the center of the disc at equal radial distances from the center of the disc. Correspondingly disposed openings in the electrodes 60, 62 and 64 and tubes 66, 68 and 70 are arranged in series alignment with each other.

The electrodes 60, 62 and 64 are separated by rings 82, 84, 86, 88 and 90 of insulating material, which preferably is a refractory, ceramic material. The rings 82 are disposed around the central openings 72 of the various electrodes. The rings 86, 88 and 90 are respectively disposed around the openings 74, 78 and 80. The electrodes may be brazed to the rings to provide vacuum tight seals and to form five channels 92, 94 and 96, 98 and 100 for the ion beam and the beams of fast neutral particles.

Five ion sources are provided one for each of the channels 92, 94, 96, 98 and 100. Three of these sources 102, 104 and 106 are shown in FIG. 5. These ion sources may be attached by vacuum tight seals to the electrode 60 and provide positive ion beams which enter the channels 92, 94, 96, 98 and 100. Nozzles 108, attached to the ion sources for .admitting gas to be ionized into these sources 110, are provided in the rings 82, 84, 86, 88 and 90 between the first and second electrodes 60 and 62 for admitting the neutral charge exchange gas into the channels 92, 94, 96, 98 and 100. Sources of operating voltage may be connected to the electrodes 60, 62, and 64 and to the ion sources. A single source of potential may be used to operate the apparatus and to provide an accelerating field which facilitates the multiple charge exchanges in each of the channels 92, 94, 96, 98 and 100. Each channel operates to provide a beam of neutral particles, as explained in connection with FIG. 1. While five channels are shown in the apparatus of FIGS. and 6, more or less channels may be provided by forming a larger or smaller number of openings on the electrode discs. The thrust provided by the rocket engine may therefore be achieved as desired.

From the foregoing description, it will be apparent that there has been provided an improved method of and apparatus for a particle acceleration whereby a beam or beams of neutral particles may be emitted. Limitation due to space charge effects, and the need for ion neutralization in the emerging beam are substantially eliminated, since neutral particles are emitted in accordance with the teachings of the invention. The efiiciency of the apparatus embodying the invention is also relatively high as compared to the efficiency of known particle acceleration and propulsion devices. The amount of energy which is expended to produce ions to obtain a beam of given density is diminished because the cascade effect results in the emission of many fast neutrals for eachion.

Variations and modifications in the herein illustrated embodiment of the invention, for example, a .neutral molecular gas, such as hydrogen or nitrogen, and ionized molecules of that gas may be used in the illustrated devices, will undoubtedly suggest themselves to those skilled in the art. Thus the foregoing description should be taken merely as illustrative and not in any limiting sense.

What is claimed is:

1. The method of providing a stream of fast particles 2. An atom accelerator comprising (a) means for precipitating a plurality of successive charge exchanges between ions and atoms of the same element, thereby producing fast atoms and slow ions, and

(b) means for accelerating said slow ions between successive ones of said charge exchanges, said last named means serving to produce a flow of neutral particles.

3. Apparatus for producing from a gas of atoms a beam of fast neutral atoms, said apparatus comprising (a) a first structure having an outlet,

(b) means for admitting said gas into said structure,

(c) a second structure for developing a field for accelerating ions along a path through said first structure toward the outlet thereof, said second structure including a plurality of electrodes, and

(d) means for injecting ions into said second structure to initiate a plurality of successive ion-atom charge exchanges successively spaced along said path to provide a beam of fast neutral atoms.

4. Apparatus for producing a beam of neutral atoms comprising (a) an evacuable envelope,

(b) an electrode structure including a plurality of spaced electrodes located in said envelope, said electrode structure defining a channel,

(c) means for introducing a gas of said neutral atoms into said envelope so that said gas enters said channel,

(d) a source of ions communicating with said envelope for introducing ions of the same species as the neutral atoms of said gas into said channel, and

(e) means for applying voltages to said electrodes to establish an ion accelerating electric field along said channel so as to support a plurality of successive ion-atom charge exchanges along said channel which result in a beam of neutral atoms in said channel.

5. Apparatus for accelerating atoms comprising (a) a plurality of electrodes each having an aperture therethrough, said electrodes being spaced from each other with their apertures aligned in series with each other to define a channel,

(b) an evacuable structure enclosing said channel,

(c) means for introducing a gas of neutral atoms into said channel for charge exchange therein with ions,

(d) an ion source communicating with said channel for injecting ions of the same species as said atoms into one end of said channel, and

(e) means for applying successively larger voltages to different ones of said electrodes spaced successively along said channel starting from said ion injection end of said channel for supporting a plurality of successive charge exchanges in said channel and thereby providing a plurality of accelerated atoms.

6. A propulsion device comprising (a) a plurality of centrally apertured disc electrodes arranged coaxially with each other in spaced relationship,

(b) a plurality of rings of insulating material disposed between adjacent ones of said electrodes and secured thereto in vacuum tight relationship to define an evacuable structure, the apertures in said electrodes defining an axial channel through said structure,

(c) an ion source secured to said structure for injecting ions into one end of said channel,

(d) an inlet extending through one of said rings for introducing a gas of the same species as said ions into said channel, and

(e) means for applying voltages of successively increasing magnitude respectively to different ones of said electrodes starting at said electrode adjacent to said ion source whereby to support a plurality of 9 successive charge exchanges in said channel to produce a beam of fast atoms.

'7. A rocket engine for providing an exhaust of atoms of a certain element comprising (a) a plurality of centrally apertured disc electrodes arranged coaxially with each other and spaced from each other a distance approximately equal to the mean free path for charge exchange between ions and atoms of said element,

(b) a plurality of rings of ceramic material disposed between adjacent ones of said electrodes and secured thereto in vacuum tight relationship to define an evacuable structure,

() said apertures defining an axial channel through said structure,

(d) means for injecting positive ions of said element into one end of said channel,

(e) an inlet extending through one of said rings for introducing a neutral gas of said element into said channel, and p (f) means for applying negative voltages of successively increasing magnitude respectively to different ones of said electrodes starting at said electrode adjacent to said ion injection end whereby to support a plurality of successive charge exchanges in said channel to emit a beam of said atoms from said channel.

8. A rocket engine for providing an exhaust of atoms of a certain element comprising (a) a plurality of centrally apertured disk electrodes arranged coaxially with each other and spaced from each other a distance approximately equal to the mean free path for charge exchange between ions and atoms of said element,

(b) a plurality of rings of ceramic material disposed between adjacent ones of said electrodes and secured thereto in vacuum tight relationship to define an evacuable structure,

(c) said apertures defining an axial channel through said structure,

(d) means for injecting positive ions of said element into one end of said channel,

(e) an inlet extending through one of said rings for introducing a neutral gas of said element into said channel,

(f) a tube connected to the end of said channel opposite said ion injection end for inhibiting the diffusion of said gas out of said channel, and

(g) means for applying negative voltages of successively increasing magnitude respectively to different ones of said electrodes starting at said electrode adjacent to said ion injection end whereby to support li a plurality of successive charge exchanges in said channel to emit a beam of said atoms out of said tube.

9. A rocket engine comprising a plurality of plates each having a plurality of similarly disposed apertures therein, said plates being spaced from each other with their similarly disposed apertures separately arranged in series alignment to define a plurality of channels, a plurality of spacer rings disposed between adjacent ones of said plates, each of said rings being disposed around different ones of said apertures, said rings and said plates defining a plurality of evacuable structures, means for introducing a charge exchange gas of neutral atoms separately into each of said structures, means for introducing ions separately into different ones of said channels through the apertures in one of said plates, and means for applying voltages of successively higher magnitudes separately to successive ones of said plates starting at the one of said plates through the apertures of which said ions are introduced.

10. Apparatus for emitting neutral particles in a given direction comprising means for providing a mixture of charged and neutral particles,

means for accelerating said charged particles in said direction to cause charge exchanges to take place between said charged particles and said neutral particles whereby further charged particles and neutral particles result, and

means for accelerating said further charged particles in said direction to cause successive charge exchanges to take place between said further charged particles and said neutral particles,

said charge exchanges resulting in the emission of a plurality of neutral particles from said mixture.

References Cited by the Examiner UNITED STATES PATENTS 2,460,175 1/ 1949 Hergenrother 230-69 2,880,337 3/1959 Langmuir et al 35.5 3,075,115 1/1963 Flowers et a1. 6035.5 3,122,882 3/1964 Schultz et a1. 6035.5 3,136,908 6/1964 Weinman BIS-63 OTHER REFERENCES Aviation Week, Oct. 31, 1960, pp. 72, 73, 74, 76, 77 and 79.

MARK M. NEWMAN, Primary Examiner. SAMUEL LEVINE, Examiner.

C. R. CROYLE, Assistant Examiner. 

6. A PROPULSION DEVICE COMPRISING (A) A PLURALITY OF CENTRALLY APERTURED DISC ELECTRODES ARRANGED COAXIALLY WITH EACH OTHER IN SPACED RELATIONSHIP, (B) A PLURALITY OF RINGS OF INSULATING MATERIAL DISPOSED BETWEEN ADJACENT ONES OF SAID ELECTRODES AND SECURED THERETO IN VACUUM TIGHT RELATIONSHIP TO DEFINE AN EVACUABLE STRUCTURE, THE APERTURES IN SAID ELECTRODES DEFINING AN AXIAL CHANNEL THROUGH SAID STRUCTURE, (C) AN ION SOURCE SECURED TO SAID STRUCTURE FOR INJECTING IONS INTO ONE END OF SAID CHANNEL, (D) AN INLET EXTENDING THROUGH ONE OF SAID RINGS FOR INTRODUCING A GAS OF THE SAME SPECIES AS SAID IONS INTO SAID CHANNEL, AND (E) MEANS FOR APPLYING VOLTAGES OF SUCCESSIVELY INCREASING MAGNITUDE RESPECTIVELY TO DIFFERENT ONES OF SAID ELECTRODES STARTING AT SAID ELECTRODE ADJACENT TO SAID ION SOURCE WHEREBY TO SUPPORT A PLURALITY OF SUCCESSIVE CHARGE EXCHANGES IN SAID CHANNEL TO PRODUCE A BEAM OF FAST ATOMS. 